The solution of ion analysis problems by liquid chromatography is often complicated by limitations in the capability to detect the eluted sample ions in the column effluent. An example is the problem of analyzing the many inorganic and organic ions that are non-light-absorbing. Whereas the seperation of such "transparent" ions may be conveniently effected on ion exchange resin columns, their detection and measurement by conventional photometric means is ineffective since they are optically indistinguishable from the transparent eluents commonly prescribed by the art. Hence, current and conventional practice teaches that photometers are useful only where the ions to be determined either contain chromophores or can generate chromophores through post-column reactions with appropriate reagents.
A second field of background art of interest to the invention is Ion Chromatography (IC). Ion Chromatography comprises a two column arrangement followed by a conductance detector. The first column serves to separate the ions of interest while the second column, "the suppressor", serves to lower the conductance of the eluent while not significantly affecting the conductance of the eluted sample species. The suppressor column in IC disadvantageously becomes exhausted in the course of normal usage and must be periodically regenerated or replaced--usually regenerated. Whereas the regeneration step has been automated in commercial instruments so that it is less obtrusive, it would be desirable to develop sensitive single column (suppressorless) methods for the many ions that lack chromophores for the following reasons:
(1) Reduced complexity of the instrumentation and in its operation. This is a very important factor in the process control area where the demands for unattended and relatively maintenance-free operation are most essential;
(2) Faster analysis times due to void space elimination, and less loss of resolution since the suppressor column contributes to peak broadening;
(3) Less alteration in calibration with usage. In Ion Chromatography, such changes can arise for ions whose elution behavior depends on the degree of exhaustion of the suppressor;
(4) Elimination of detrimental suppressor column side reactions, with resulting broader sample species applications; and
(5) Eluent recycle capability and elimination of suppressor column regenerant.
There are other problem areas which the invention addresses. Of particular importance is the ability to determine ions by flow analysis for which past IC and photometric methods show relatively poor sensitivity. Carbonate determination is an example that is demonstrated specifically herein, and for which both IC and photometric methods are generally not ideally suited, or are unsuited for such analysis.
Another significant improvement is achieved in the area of calibration procedures. Most IC methods, as well as most methods of analysis, generally rely on a calibration procedure wherein the user establishes experimentally the relationship between peak height or peak area response and the amount of sample species injected. Each species has a response factor which is characteristic for that species and usually different from that of other species. This requires that response factors be determined for all species of interest. In contrast, in practicing the invention, it has been discovered that a number of ions will have a common response factor. This obviously and beneficially simplifies instrument operation.